The invention relates to novel rhodium catalysts and also to a process for their preparation. These novel rhodium catalysts are suitable in particular for the preparation of substituted or unsubstituted diphenylamine from appropriately substituted dicyclohexylamine, as a result of which the invention furthermore relates to their use in a process for the preparation of this substituted or unsubstituted diphenylamine by using rhodium catalysts.
DE-OS (German Published Specification) 2,331,878 discloses a process for the preparation of diphenylamine and derivatives thereof, in which the starting materials are imines such as N-cyclohexylidene-aniline and derivatives thereof which are dehydrogenated in the gas phase in the presence of supported catalysts based on nickel, platinum, palladium or copper/chromium. N-cyclohexylidene-aniline, for example, is prepared from cyclohexanone and aniline by condensation.
Furthermore, DE-OS (German Published Specification) 2,520,893 discloses to prepare diphenylamine by catalytic dehydrogenation of compounds and/or compound mixtures consisting entirely or partly of hydrogenated diphenylamine in the presence of a dehydrogenation catalyst containing nickel/chromium, aluminium, copper, manganese and alkali. Compounds of this type shown in the working examples are binuclear aromatic imines.
A further process known from Kinet, Katal. 28 (1), 250-254 (quoted in C.A. 107 (23), 217420 z) finally shows that in addition to the dehydrogenating aromatization a cyclization to the carbazole also has to be taken into account when using hydrogenation/dehydrogenation catalysts. For example, using Pt (1%)/Al.sub.2 O.sub.3 at 380.degree. C., N-cyclohexylaniline yields 41% of carbazole and only 18% of diphenylamine; under similar conditions, dicyclohexylamine yields 41% of carbazole and onlt 17% diphenylamine, and N-cyclohexylidene-aniline yields 40% of carbazole and 20% of diphenylamine. In addition, deamination and rearrangement products such as aniline, benzene, diphenyl and 4-amino-diphenyl are observed.
The processes mentioned yield insufficient conversions and yields and as a process to be carried out industrially they are in need of improvement in every respect. The desire to provide a process to be carried out industrially in a simple manner and ensuring high yields and allowing easy work-up of the reaction products is satisfied by the invention.